1. Field of the Invention
The present invention relates to an image reading apparatus that senses from above the open-faced surfaces of a document such as a book or the like, and more specifically relates to an image reading apparatus which accomplishes focusing and corrects image distortion by detecting the distance to a document surface.
2. Description of the Related Art
Conventional image reading apparatuses that sense from above the open-faced surfaces of a document disposed face upward are known which sense, in addition to sensing the surface of the document, the image of the side surface of a document, e.g., the image of the interior side surface of the document, and recognize the shape of the document side surface by detecting the difference in the luminance of said side surface and the luminance of the background so as to detect the height of the document from the detected shape of said side surface, and thereby accomplish focusing in accordance with the height of the document as well as correct image distortion caused by changes in the height of the document.
Therefore, the height detection methods of the aforesaid conventional apparatuses are disadvantageous in that when there is a difference in the heights of the interior side and the front side of a document, focusing and image distortion correction are inadequate because the height of the front side of the document cannot be accurately measured since the shape of the document side surface is recognized by sensing the side surface of the interior side of the document. Examples of such situations include, as shown in FIGS. 17a and 17b, situations when a document is placed so as to form a peaked shape without the interior side of the document being adequately open and when a document is placed so as to form an inverted peaked shape without the front side of the document being adequately open, situations when page distortion is caused by the poor condition of the document as shown in FIG. 17c, and situations when a document is placed so as to be inclined as shown in FIG. 17d.